Energy efficiency isn’t as sexy as inventing new, cleaner forms of power. But, if you care about climate change, you really ought to care about it. Efficiency will need to account for a third of emissions reductions by 2040 if we’re stay within relatively safe global warming limits, according to the International Energy Agency.

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“Simply put, there are no realistic or affordable energy and climate change policy without a sizable and vigorous energy efficiency component,” the IEA says in a new report.

The good news is we are becoming smarter about energy use. Energy “intensity”–the amount of energy needed to generate a unit of global GDP–improved by 1.8% last year. That was higher than in 2014, even though energy prices have been falling, which normally encourages people to use more energy, not less.

Between 2000 and 2015, IEA countries–including the U.S. and most of Europe–improved their energy intensity by an average of 14%. That’s the equivalent of 450 million tonnes of oil, or enough to power Japan for a whole year. In all, efficiency saved $490 per person across the IEA area, or cumulative energy spending of $4 trillion.

The bad news is that even this rate isn’t enough to meet the 2040 target. The IEA calls on countries to implement more mandatory efficiency targets–like our CAFE car efficiency standards–and to spread more efficient lighting, heating, and air conditioning technology. Best-in-class equipment could save 14% of global residential energy consumption, the report says.